The present invention relates to combined cycle electric power plants and, more particularly, to control apparatus for regulating the position of a variable inlet guide vane assembly associated with the gas turbines employed in such plants. Further, such regulation is accomplished as a function of gas turbine compressor inlet temperature in accordance with desired gas turbine exhaust flow at predetermined operating conditions.
Several control arrangements for combined cycle power plants or for gas turbines have included control means for adjusting the inlet guide vanes of a gas turbine as a function of a predetermined gas turbine parameter. One example of such prior art arrangements is U.S. Pat. No. 3,087,486, issued to R. Roc on July 16, 1963, wherein air flow to the compressor of a gas turbine is varied in a combined cycle power plant in response to changing power demand. Another example of such prior art arrangements can be found in U.S. Pat. No. 3,623,326, issued to C. Greune on Oct. 30, 1971, wherein guide vane regulation of a gas turbine is accomplished for a vehicle as a function of gas temperature downstream from the combustion chamber.
Also of interest is U.S. Pat. No. 2,946,187 issued on July 26, 1960 to R. Zorschak et al. While no particular inlet guide vane positioning control is disclosed therein, this patent does allude to the fact that there is a need in combined cycle power plants to respond to temperature changes at the gas turbine inlet in order to promote and improve overall plant efficiency. Unfortunately, no particularized solution is presented herein or in any of the other prior art arrangements.
In a combined cycle power plant as envisioned herein, it is a requirement that the associated gas turbine, steam generator and steam turbine be properly matched in order to optimize overall plant efficiency and prevent down time. With respect to such a requirement, one item that calls for special attention is that of gas turbine exhaust flow. Generally, the gas turbine compressor operates at constant speed and compresses essentially a constant air volume. This produces a variable gas turbine exhaust flow, which varies as a function of changes in inlet conditions. Over the normal operating range, changes in the inlet temperature from -40.degree.F to +120.degree.F cause an approximate 30% change in gas turbine exhaust flow.
The quantity of steam and the steam temperature delivered by the steam generator to the steam turbine are determined by the exhaust flow from the gas turbine and the amount of afterburner firing. For a constant gas turbine exhaust flow and temperature, increasing afterburner firing rate will increase both the steam flow and temperature. For a constant afterburner firing rate, increasing the gas turbine exhaust flow will increase the steam flow and decrease the steam temperature.
As the gas turbine inlet temperature decreases, the gas turbine exhaust flow increases. If the steam temperature is to be held constant, the afterburner firing rate must be increased with the resulting increase in steam flow. If the afterburner is not increased, the steam flow will increase and the steam temperature will decrease. The steam turbine is designed for a certain steam flow at a specified pressure and temperature. Since the PACE 260 steam turbine is designed to operate in the turbine following mode (control valves open), the steam pressure must be increased in order to increase steam flow. However, reducing steam temperature results in moisture problems in the steam turbine.
The three sets of conditions briefly described above, indicate that one solution to the problem is to limit the gas turbine exhaust gas flow corresponding to operation with the inlet guide vanes open, steam temperature at 950.degree.F, the superheater bypass valve fully closed and the steam turbine max. flow limited. This condition exists at an inlet ambient temperature of approximately +40.degree.F. If the gas turbine exhaust flow is maintained at a valve corresponding to these conditions, as the inlet ambient temperature decreases, the plant will operate properly at the maximum power generation rate. The gas turbine exhaust flow can be reduced by partially closing the inlet guide vanes. A solution to controlling the position of the inlet guide vanes to produce a maximum gas turbine exhaust flow rate at all ambient temperature conditions is to limit the inlet guide vane position as a function of gas turbine inlet temperature.